When opticians, for optically adapting spectacles, determine the centering data, that is, all required information for the correct centering of spectacle lenses in a spectacle frame for the individual use purpose of the customer or subject, then the following procedure can be adopted:
The customer is requested to adopt a head and body posture that is as natural as possible. The centering values are then defined by marking the viewing points in the disc plane of the spectacle frame by means of a pen or by means of video recording and subsequent manual centering data determination (such as in the case of the video centering device sold by the applicant under the trade name “Video Infral”) or automated centering data determination (such as in the case of the video centering device sold by the applicant under the trade name “RV terminal”).
In order to determine the forward inclination angle, an aid called “Y-Stick” is known, which “freezes” the forward inclination angle of the spectacle frame at a point in time in order that the angle can subsequently be read. The forward inclination angle thus detected is used in a subsequent step for positioning the customer in the same position in front of a centering system.
Furthermore, EP 1 591 064 A1 discloses a measuring instrument for determining the inclination of the head, this being sold by the applicant of EP 1 591 064 under the designation “Ysis”. The measuring instrument is fixed to the spectacle frame. This proposal aims to capture the “normal” head posture at a point in time in each case for the near and far viewing directions. From the head inclinations assumed when looking into the distance and when reading, the difference in head inclination between distant-vision and close-vision tasks is determined. The associated viewing points through the spectacle lens are then determined therefrom.
All the above methods are static, that is to say that a “snapshot” of the respective situation is used for determining a habitual head posture.
U.S. Pat. No. 6,827,443 discloses classifying the behavior of a wearer of spectacles with regard to the latter's viewing habits. The typical head and eye movement is determined for various vision tasks. Depending on that, the viewing behavior of the subject is categorized according to “mobility” of the eyes and of the head (“head/eye mover”). The results of the analysis are used in this case as a basis for selecting a suitable spectacle lens.
United States patent application publication 2007/0229761 discloses ascertaining the most frequent viewing directions relative to the head. For this purpose, head posture and viewing direction are detected simultaneously. The relative “net” viewing direction and the resultant viewing point through a spectacle lens are determined from the relative viewing directions determined.
DE 10 2004 063 160 A1 describes a method and a device for adapting spectacles, in particular for detecting the habitual head posture of a subject, the measurement data of which are incorporated in video centering data determination. The method involves taking a number of video photographs of the subject in order to determine the head posture angles of a subject in various head posture positions, detecting the head posture angles and adjusting them with the forward inclination angle of the spectacle frame and also the corneal vertex distance. The corresponding video photographs are captured at a moment when, in the operator's opinion, the subject has adopted his or her natural habitual head posture.
A centering measurement, that is, a measurement which involves determining all the required information for the correct centering of spectacle lenses in a spectacle frame for the individual use purpose of the customer or subject, constitutes a forced situation for the customer of the optician. Although it is known from the centering measurement how the customer holds his or her head during a recording, this head posture does not necessarily correspond to the “normal”, that is, the most frequent head posture in actually unforced situations.
This also does not change if the method described in DE 10 2004 063 160 A1 is used. Furthermore, this method entails the risk of the purportedly natural head posture not being recorded at all, but rather a different head posture, since the subject has already adopted a different head posture within the reaction time of the operator.